Constant engagement tool path generation for convex geometries

Abstract

Contour-parallel and direction-parallel machining strategies have gained nearly universal acceptance for simple 212D material removal tasks. When the mechanics of the milling process are considered, widely varying cutter engagements and sharp velocity discontinuities are found to significantly limit the efficiency of these approaches. To address these problems, a new offset for tool path generation that maintains an ideal steady-state process by explicitly controlling the instantaneous engagement of the cutting tool is introduced. A generative spiral-in algorithm for 212D material removal of convex geometries is presented that continuously monitors the state of the milling process, including cutter engagement, path curvature, and the instantaneous entry and exit angles. A smooth (C2) trajectory is generated, maintaining an ideal constant engagement state as much as possible. During periods in which constant engagement cannot be maintained, the path curvature and entry and exit angles are controlled explicitly. A state-based algorithm is developed that formalizes these alternate strategies of path generation and controls the transitions between them. Case studies are shown comparing these new tool path trajectories with conventional contour-parallel approaches. Simulations reveal far less variation in cutter engagement and near-constant material removal rates. Overall path lengths are shorter, and acceleration requirements (as dictated by path curvature) are greatly reduced. Experimental data from a three-axis dynamometer reveals an expected close correlation between instantaneous engagement and cutting force. Potential benefits include reduced tool chipping and breakage, reduced cycle times, increased work-piece quality, and reduced machine fatigue and wear. In the domain of high-speed machining, acceleration and deceleration often constitute a significant component of the overall processing time, further emphasizing the importance of a smooth tool path trajectory.